GSP was finished twice in his prime athletic years (25-30, after that you're old, which is why only 5% of Olympic gold medals go to guys over 30, only 10 of NBA MVP's over 30 etc): Hughes and Serra. Anderson was finished twice in his prime athletic years as well: Takase and Chonan. The "invincibility" of both Anderson and GSP is smoke and mirrors -- their fans simply say any fight they lost was because they weren't in their prime. That becomes a tautology, a "no true Scotsman" type of thing.
Hendricks was only GSP's fourth toughest fight. His two toughest fights were Serra and Hughes (they both finished him, that's as bad as it gets). GSP's third toughest fight was the first Penn fight. GSP had to go straight to the hospital after that fight (no press conference), unlike the Hendricks fight in which he had no trouble going to the press conference. Beats me why Hendricks gets so much credit (he should have won 48-47, but it came down to one round), and Hughes, Serra and Penn so little for doing worse to him (two finished him, and one hospitalized him). And right now it looks like its Bisping who retired GSP.
The only fighter who hasn't lost is Jones, though he's had two very close calls (the first Gus fight, and Reyes). The fighter who has the most impressive career is Fedor, simply because he never cut weight to avoid other fighters. Anyone who cuts weight is basically saying there are bigger guys who they don't want to fight.
To me, giving so much importance to the "athletic prime" factor is dumb. First of all, is far from rigurous or scientific with the data you have, but anyways, fighting is too complex to say age is the critical factor in a fighter performance. There are plenty of mma champions in his mid even late 30s. Not to mention nowadays an athlete in his 30s can have an excellent physical conditioning, even beter than athletes in their 20s from 20 years ago
The weight cutting argument for the most impressive career is dumb too, in my opinion. Its no coincidence the only one who didnt cut weight is the HW. He got KOes by a way smaller, older man than him in Dan Henderson by the way. An also way smaller man in Arona took him to a very close judge decision.
Everyone you mentioned have a case for greatest of all time, if thats what your point was. As far as I see it:
#1 Anderson - Set records for finishes in tittle fights. Beat top contenders at 170, 185 and 205. Even in the loss, to take Cormier and Adesanya to a decision being in his 40s, after 20 years of pro career with bad injuries, is GOAT material as well imo.
#2 Jones - Longest run as a champion and invincibility. Not #1, in my opinion, because only proved himself in one division, usually with notable size advantage, has had very close decisions with Santos, Reyes and Alex, and his best win has been rules as NC for dopping.
# 3 GSP - Most number of defenses in UFC only after Jones. Beat lot of solid fighters, barely losing a round. His comeback for inishing MW champion Bisping is epic. Still not the best imo because always refused to test himself in other divisions until his last fight, despite he faced many LWs moving up. His lack of finishing ability, compared to Silva and Jones is a big reason for not being considered the GOAT in this sport imo.
Behind them, I think Cormier and DJ have the best case (Khabib and Conor are on their way). Fedor has as well as the best from the past era; and I guess no fighter from the 90s gets to be in this discussion.